1. Field of the Invention
The present invention relates to a process for neutralizing and reducing the residual halogen content in alkoxysilanes or alkoxysilane-based compositions.
2. Discussion of the Background
Alkoxysilanes and alkoxysilane-based compositions are used in many areas as adhesion promoters, crosslinkers in polymers, as release agents, as additives in paints and coatings, for rendering surfaces hydrophobic, for textiles and leather, protecting buildings and facades, book conservation, modification of surface properties, coating glass fibers or silanizing filters and pigments, and also for improving the Theological properties of polymer dispersions and emulsions.
Alkoxysilanes are generally prepared using halosilanes, and particularly chlorosilanes. For example, reacting chloropropyltrichlorosilane with methanol produces a chloropropyltrimethoxysilane with the elimination of HCI. Residual halogen content, i.e. residues of acidic or hydrolyzable chloride, can remain in the alkoxysilane, even after purification of the product by distillation. In addition, residual amounts of nonhydrolyzable chlorine can also remain in the product, for example, in the preparation of aminoalkylalkoxysilanes, due to unreacted starting materials such as chloroalkylalkoxysilanes.
Attempts are currently being made to prepare alkoxysilanes and products which contain alkoxysilanes (e.g. EP 0 273 867 B1, DE 27 51 714 C2) and alkoxysilane-based compositions (for example, EP 0 049 365 A2, EP 0 518 057 A1, EP 0 675 128 A1, EP 0 716 127 A2 or EP 0 716 128 A2, possible, particularly acidic or hydrolyzable chlorides. An essential product characteristic is color, and Gardner/APHA color indexes, as specified in ISO 6271, that are as low as possible are being sought.
It is known to remove the residual halogen content from alkoxysilanes by reacting or neutralizing the residual halogen content with alkali metal alkoxides, such as sodium methoxide, and separating off the resulting salt (EP 0 282 846 A2, EP 0 741 137 A1). Neutralization processes of this type have the disadvantage that considerable amounts of product are also decomposed in the reaction. Decomposition products include siloxanes, tetralkoxysilane, and addition products such as methoxyethyltrimethoxysilanes by reaction of sodium methoxide and vinyltrimethoxysilane, methoxypropyl-trimethoxysilane formed from sodium ethoxide and chloropropyltrimethoxysilane, methoxypropylmethyl-dimethoxysilanes from sodium methoxide and chloropropylmethyldimethoxysilane, or, in the case of the reaction of alkali metal methoxide and an alkyltrimethoxysilane, the highly toxic tetramethoxysilane.
In addition, when the usual neutralizing agents, such as alkali metal alkoxides, are used, the color index of the treated alkoxysilane is frequently impaired.
The publications, CN 11 07 85 1A and 11 07 85 2 A, describe processes for preparing vinyl-triethoxysilane and chloropropyltrimethoxysilane, which are said to provide very pure products by using magnesium alkoxides. The disadvantage of these processes is that the magnesium alkoxides, being highly moisture-sensitive products, are difficult to handle. Furthermore, magnesium alkoxides are present in powder form and have a tendency to form lumps, which again makes it difficult to meter the magnesium alkoxide exactly. This disadvantage particularly affects the metering into heated or hot products, which is usually carried out in practice, because the silane and other volatile constituents of the reaction preparation condense on the magnesium alkoxides, partially dissolving them and thus converting them into a form no longer flows.
To avoid the above-noted disadvantage, attempts have been made to use alcoholic solutions of the magnesium alkoxides. However, such attempts are burdened by the problem that considerable amounts of alcohol are added in the neutralization step and must be separated off again by distillation, which requires both time and energy. In addition, the magnesium alkoxides are highly expensive and are available only with restrictions in industrial amounts.